The present invention relates to a presensitized plate requiring no dampening water which is suitable for use in making a lithographic printing plate which makes it possible to perform printing operations without using any dampening water.
There have been proposed various presensitized plate requiring no dampening water (hereinafter referrd to as "PS plate requiring no dampening water") for use in making a lithographic printing plate for performing lithographic printing without using any dampening water (hereinafter referred to as "lithographic printing plate requiring no dampening water"). Among these, PS plates requiring no dampening water which comprise a substrate provided thereon with, in order, a light-sensitive resin layer and a silicone rubber layer exhibit very excellent quality and are disclosed in, for instance, Japanese Patent Publication for Opposition Purpose (hereinafter referred to as "J.P. KOKOKU") Nos. Sho 54-26923 (=U.S. Pat. No. 3,894,873) and Sho 55-22781 (=British Patent No. 1,419,643).
The silicone rubber layer used in these PS plates requiring no dampening water have currently been those obtained by partially crosslinking polymers comprising, as the main skeleton, polysiloxane with crosslinking agents. The following two methods have usually been adopted for hardening the silicone rubber layer.
(1) Condensed Silicone Rubber Layer: An organopolysiloxane having hydroxyl groups at both ends is crosslinked with a silane or siloxane carrying a hydrolyzable functional group directly bonded to the silicon atom thereof to give a silicone rubber.
(2) Addition Type Silicone Rubber Layer: A polysiloxane having .tbd.Si--H groups and a polysiloxane having --CH.dbd.CH-- groups are subjected to an addition reaction to give a silicone rubber (see, for instance, Japanese Unexamined Patent Publication (hereinafter referred to as "J.P. KOKAI") No. Sho 61-73156 and No. Hei 3-161753).
The condensed silicone rubber (1) changes its curing properties and adhesion to a light-sensitive layer depending on the moisture content of the atmosphere during hardening and, therefore, it suffers from the problems that if it is used for preparing a PS plate requiring no dampening water, the sensitivity thereof easily varies and that the stable production thereof is very difficult. For this reason, it is considered that the addition type silicone rubber is rather excellent since it does not suffer from such drawbacks.
There have been used, as a light-sensitive resin layer for positive-working PS plates, those obtained from a photopolymerizable light-sensitive composition which is hardened through exposure to light. In the PS plate having such a layer structure, images are in general formed by exposing it to light for hardening the light-sensitive resin layer, or optionally photoadhering the light-sensitive layer to the silicone rubber layer as the upper most layer at the interface therebetween to thus firmly link these layers and prevent the permeation of a developer into the exposed region and the leaching of the light-sensitive resin layer. A non-image area comprising the silicone rubber layer is thus formed. On the other hand, an image area is formed by permeating a developer into the unhardened light-sensitive resin layer through the silicone rubber layer to partially or completely dissolve out the unhardened light-sensitive resin layer and then physically removing the silicone rubber layer thereon. If images are formed in such a manner and the hardening of the silicone rubber layer is insufficient, the silicone rubber layer of the resulting lithographic printing plate wears out during printing or it has a low adhesion to the light-sensitive layer and consequently the printing plate does not exhibit sufficient printing durability.